It's a difficult one, maybe Andy could find another doubles partner and let Jamie play in the mixed not just for the olympics. I think the doubles are good for co-ordination and net skills. Yes Colin and Ross are playing very well and should be GB reps. Also agree Nigel that Heather Watson has talent and a great fighting spirit.
As i understand it, there's a reasonable chance of having two men's doubles teams in the Olympics, Stuart Fraser has done a piece about qualification and says "eight places, effectively wild cards, will be allocated by the ITF. Great Britain will be hopeful of receiving one of these places if either Andy and Jamie Murray or Colin Fleming and Ross Hutchins fail to qualify by right".
Also, for mixed doubles: "Rather than entries being done in advance like the other events, entries shall be made by each national association on-site. Players are only eligible for the Mixed Doubles if they are competing in the other events, which means that someone like Jamie Murray could not compete in the mixed if he was not playing in the Men’s Doubles".
Lots more herehttp://talkingballs-tennis.blogspot.co.uk/2012/03/guide-to-olympics-qualification.html#comment-form
(2nd comment/question at the end was mine!)